CHAP. V 
NATURAL SELECTION 
103 
a state of nature. We have now to inquire whether there is 
any analogous process in nature, by which wild animals and 
plants can be permanently modified and new races or new 
species produced. 
Effect of Struggle for Existence under Unchanged Conditions. 
Let us first consider what will be the effect of the struggle 
for existence upon the animals and plants which we see around 
us, under conditions which do not perceptibly vary from year 
to year or from century to century. We have seen that every 
species is exposed to numerous and varied dangers throughout 
its entire existence, and that it is only by means of the exact 
adaptation of its organisation — including its instincts and habits 
—to its surroundings that it is enabled to live till it produces 
offspring which may take its place when it ceases to exist. 
We have seen also that, of the whole annual increase only a 
very small fraction survives; and though the survival in indi¬ 
vidual cases may sometimes be due rather to accident than 
to any real superiority, yet we cannot doubt that, in the long 
run, those survive which are best fitted by their perfect organisa¬ 
tion to escape the dangers that surround them. This “ survival 
of the fittest ” is what Darwin termed “ natural selection,” 
because it leads to the same results in nature as are produced 
by man’s selection among domestic animals and cultivated 
plants. Its primary effect will, clearly, be to keep each species 
in the most perfect health and vigour, with every part of its 
organisation in full harmony with the conditions of its existence. 
It prevents any possible deterioration in the organic world, and 
produces that appearance of exuberant life and enjoyment, of 
health and beauty, that affords us so much pleasure, and which 
might lead a superficial observer to suppose that peace and 
quietude reigned throughout nature. 
The Effect under changed Conditions. 
But the very same process which, so long as conditions re¬ 
main substantially the same, secures the continuance of each 
species of animal or plant in its full perfection, will usually, 
under changed conditions, bring about whatever change of 
structure or habits may lie necessitated by them. The changed 
conditions to which we refer are such as we know have occurred 
